Transposition of the Great Arteries
Definition
Transposition of the Great Arteries (TGA), also known as Dextro transposition, occurs when aorta and pulmonary artery are misplaced in relation to the ventricular septum, with aorta rising from the right ventricle and pulmonary artery rising from the left ventricle.
When the great arteries have the normal origins and are only spatially misplaced, tney are said to be malposed, also known as congenitally corrected TGA. In this particular malformation the ventricles are on the opposite sides of the heart to the usual. The atria are connected to the incorrect ventricle and the great arteries also come from the wrong ventricles (transposition).
Pathology
The abnormal relationship is between the ventricles and the great arteries, both atria and ventricles are present in their normal anatomical positions. Both inferior vena cava and superior vena cava empty into the right atrium which is connected to the right ventricle and is followed by the aorta. This anatomical arrangement results in two separate and parallel circulations, Some communication between two circulations must exist in order to sustain life. If there is no shunt then unoxygenated blood flows to the systemic circulation and oxygenated blood is being shuttled to the pulmonary circulation. The majority of the patients have an interatrial communication, 2/3 of patients have a patent ductus arteriosus or a foramen ovale, and 1/3 of patients have a ventricular septal defect. 90% of neonates with TGA will die within the first year if not treated.
Hemodynamics
The majority of neonates with TGA manifest marked cyanosis within the first day of life. In addition, metabolic aidosis eveolves seconday to the poor tissue oxygenation. The degree of tissue hypoxia determines the clinical course. A bidirectional shunt is always present to prevent total volume depletion of the circulating volume. A major determining factor of systemic arterial oxygen saturation is the amount of blood exchanges between the circulations of intercirculatory shunts.
Left to right shunt is pulmonary to systemic.
Right to left shunt is systemic to pulmonary.
Systemic arterial oxygen saturation is more influenced by the left to right shunt
(LA -> RA -> RV -> Aorta).
Infants with complete TGA are susceptible to the early development of pulmonary vascular obstructive disease. Also, systemic arterial hypoxemia, increased pulmonary flows and pulmonary hypertension, all contribute to the development of pulmonary vasoconstriction.
Clinical Manifestations
The usual clinical symptoms are dyspnea and cyanosis from birth, progressive hypoxemia, and complete heart failure. Clinical manifestations are mostly influenced by the magnitude of intercirculatory mixing. The most severe cyanosis is observed in infants when only a small patent foramen ovale or ductus arteriosus are present resulting in inadequate mixing.
Medical and Surgical Interventions
Medical: limited to oxygen, digitalis, diuretics, iron (if anemia is present), and sodium bicarbonate
(for severe metabolic acidosis), prostaglandin E1 may be used for dilation of ductus arteriosus.
Palliative Procedures: enlargement ot interatrial communication may be performed by rupturing the valve of the foramen ovale by balloon catheter (Rashkind procedure). This is known as atrial septostomy and is able to sustain increased oxygen saturations for up to 9 months of age.
Surgical Interventions:
A) Atrial Switch: a creation of a new interatrial septum with a pericardial patch. As a result, systemic venous return is deverted to the left ventricle through the mitral valve, and pulmonary blood is deverted through the tricuspid valve to the right ventricle and aorta.
B) Arterial Switch: also known as Jatene and Le Compte procedures, performed by transsecting aorta and pulmonary artery and reversing these two vessels to their normal anatomical positions. This procedure, unlike atrial switch, results in the left ventricle becoming the systemic pump.
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